In many metal recovery processes, it has long been known that molten salt fluxes are effective in separating metals from their ashes. For example, in U.S. Pat. Nos. 2,987,391; 3,694,190; 3,798,024; and 3,846,123, aluminum is separated from its solid ash impurities such as its oxides, carbides, and nitrides. Metal-ash mixtures are obtained from metal recovery byproducts or waste forms such as skim, dross, metal turnings, and other forms of scrap. The utilization of molten salt fluxes, however, has been hampered by a lack of means for efficient and rapid removal of the solids which accumulate as a result of the separation process. Most of the molten metal will settle to the bottom of a container from which it is readily withdrawn. However, the accumulated ash solids and the molten salt will still contain significant amounts of the desired metal.
Various methods have been proposed for improving such metal recovery processes. For example, rotary salt furnaces have been used for separating oxides and other solids and impurities from the aluminum; but at the end of the separation process, a considerable amount of salt and metal remains with the accumulated solid impurities.
In the past, this mixture of fluxing salt, metal and solid particulate impurities, this mixture being usually referred to in the aluminum industry as "black dross," was allowed to cool in large rocks or chunks. These chunks or rocks of the black dross were usually treated with water and broken up to recover the larger pieces of aluminum metal entrained in the black dross. The metal would range in size from very fine particulates to pieces a few inches in thickness. Following removal of the mechanically recoverable aluminum metal, the remainder of the black dross was in the past discarded as having no value since it had been heretofore too expensive and difficult to recover the salt flux components or the remaining metal therefrom. In the past, the remainder of the black dross residue was disposed of by dumping it in quarries or other isolated locations. This type of disposal, however, is currently unacceptable from both a safety and ecological point of view since it presents a serious possibility that salts will be leached from this residue and could find their way into underground or surface water streams or reservoirs.
It has been proposed that the dross residue from metal recovery processes be further processed to recover substantially all of the salt flux and any remaining metal for recycle to the metal recovery process. Typically, complete recovery requires aqueous processing of the salt flux remaining in the dross residue. While such processing overcomes many of the disadvantages mentioned, the large volume of the dross residue which must be processed contributes significantly to the cost of operating a metal recovery process. Accordingly, it would be desirable to have some other method of further refining the dross residue to reduce the volume which must be subjected to aqueous processing. Further, it would be advantageous if such other process could be practiced at a temperature at which the fluxing salt was still in a molten state to reduce the thermal energy required to remelt the fluxing salt for recycle to the metal recovery process.
Another area of interest in which it is advantageous to effect such liquid-solid separation in a rapid and economical manner is in the production of magnesium from magnesium chloride. In such a process it is desirable to separate insoluble impurities from the magnesium chloride before it is used in an electrolytic production cell. The ability to effect rapid and economical separation of solids from molten salts also could be advantageously applied to metallurgical processes for the recovery of zinc, cadmium, thallium, tin, lead, antimony, and bismuth, among others.